This invention is directed to zinc oxide (ZnO) films for use in electrically excited devices such as light emitting devices (LEDs), laser diodes (LDs), field effect transistors (FETs), and photodetectors. More particularly, this invention is directed to ZnO films containing a p-type dopant for use in LEDs, LDs, FETS, and photodetectors wherein both n-type and p-type materials are required, for use as a substrate material for lattice matching to other materials in such devices, and for use as a layer for attaching electrical leads.
For some time there has been interest in producing II-VI compound wide band gap semiconductors to produce green/blue LEDs, LDs and other electrical devices. Historically, attempts to produce these devices have centered around zinc selenide (ZnSe) or gallium nitride (GaN) based technologies. However, these approaches have not been entirely satisfactory due to the short lifetime of light emission that results from defects, and defect migration, in these devices.
Recently, because ZnO has a wide direct bandgap of 3.3 eV at room temperature and provides a strong emission source of ultraviolet light, ZnO thin films on suitable supporting substrates have been proposed as new materials for light emitting devices and laser diodes. Undoped, as well as doped ZnO films generally show n-type conduction. Impurities such as aluminum and gallium in ZnO films have been studied by Hiramatsu et al. who report activity as n-type donors (Transparent Conduction Zinc Oxide Thin Films Prepared by XeCl Excimer Laser Ablation, J. vac. Sci. Technol. A 16(2), March/April 1998). Although n-type ZnO films have been available for some time, the growth of p-type ZnO films necessary to build many electrical devices requiring p-n junctions has to date been much slower in developing.
Minegishi et al. (Growth of P-Type ZnO Films by Chemical Vapor Deposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recently reported on the growth of nitrogen doped ZnO films by chemical vapor deposition and on the p-type conduction of ZnO films at room temperature. Minegishi et al. disclose the growth of p-type ZnO films on a sapphire substrate by the simultaneous addition of NH3 in carrier hydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10 mol% was used, secondary ion mass spectrometry (SIMS) confirmed the incorporation of nitrogen into the ZnO film, although the nitrogen concentration was not precisely confirmed. Although the films prepared by Minegishi et al. using a Zn/ZnO ratio of 10 mo l% appear to incorporate a small amount of nitrogen into the ZnO film and convert the conduction to p-type, the resistivity of these films is too high for application in devices such as LEDs or LDs. Also, Minegishi et al. report that the carrier density for the holes is 1.5xc3x971016 holes/cm3, which is considered to be too low for use in commercial light emitting devices or laser diodes.
Park et al. in U.S. Pat. No. 5,574,296 disclose a method of producing thin films on substrates by doping IIB-VIA semiconductors with group VA free radicals for use in electromagnetic radiation transducers. Specifically, Park et al. describe ZnSe epitaxial thin films doped with nitrogen or oxygen wherein ZnSe thin layers are grown on a GaAs substrate by molecular beam epitaxy. The doping of nitrogen or oxygen is accomplished through the use of free radical source which is incorporated into the molecular beam epitaxy system. Using nitrogen as the p-type dopant, net acceptor densities up to 4.9xc3x971017 acceptors/cm3 and resistivities less than 15 ohm-cm were measured in the ZeSe film. However, the net acceptor density is too low and the resistivity is too high for use in commercial devices such as LEDs, LDs, and FETs.
Although some progress has recently been made in the fabrication of p-type doped ZnO films which can be utilized in the formation of p-n junctions, a need still exists in the industry for ZnO films which contain higher net acceptor concentrations and possess lower resistivity values.
Among the objects of the present invention, therefore, are the provision of a ZnO film containing a high net acceptor concentration on a substrate; the provision of a process for producing ZnO films containing p-type dopants; the provision of a process for producing p-n junctions utilizing a ZnO film containing a p-type dopant; the provision of a process for producing homoepitaxial and heteroepitaxial p-n junctions utilizing a ZnO film containing a p-type dopant; and the provision of a process for cleaning a substrate prior to growing a film on the substrate.
Briefly, therefore, the present invention is directed to a ZnO film on a substrate wherein the film contains a p-type dopant. The film has a net acceptor concentration of at least about 1015 acceptors/cm3, a resistivity less than about 1 ohm-cm, and a Hall Mobility of between about 0.1 and about 50 cm2/Vs.
The invention is further directed to a process for growing a p-type ZnO film containing arsenic on a GaAs substrate. The GaAs substrate is first cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. After cleaning the temperature in the chamber is adjusted to between about 300xc2x0 C. and about 450xc2x0 C. and the excimer pulsed laser is directed onto a polycrystalline ZnO crystal to grow a film on the substrate. The temperature of the deposition chamber containing the substrate coated with the film is then increased to between about 450xc2x0 C. and about 600xc2x0 C. and the substrate is annealed for a time sufficient to diffuse arsenic atoms into the film so as to produce a net acceptor concentration of at least about 1018 acceptors/cm3 in the film.
The invention is further directed to a process for growing a p-type zinc oxide film on a substrate. The substrate is first cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. After cleaning the substrate, the temperature in the chamber is adjusted to between about 300xc2x0 C. and about 450xc2x0 C., and a p-type zinc oxide film is grown on the substrate by directing an excimer pulsed laser beam onto a pressed ZnO powder pellet containing a p-type dopant to grow a p-type zinc oxide film containing a net acceptor concentration of at least about 1018 acceptors/cm3.
The invention is further directed to a process for preparing a p-n junction having a p-type ZnO film and an n-type film wherein the net acceptor concentration is at least about 1018 acceptors/cmxe2x88x923. A substrate is loaded into a pulsed laser deposition chamber and cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. The temperature in the deposition chamber is then raised to between about 300xc2x0 C. and about 450xc2x0 C. Subsequently a p-type ZnO film having a net acceptor concentration of at least about 1018 acceptors/cm3 is grown on the substrate by directing an excimer laser onto a pressed ZnO powder pellet containing the p-type dopant. Finally an n-type film is grown on top of the p-type film by directing an excimer laser beam onto a pressed ZnO pellet containing the n-type dopant.
The invention is further directed to a process for preparing a p-n junction having a p-type ZnO film and an n-type film wherein the net acceptor concentration is at least about 1018 acceptors/cmxe2x88x923. A substrate is loaded into a pulsed laser deposition chamber and cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. The temperature in the deposition chamber is then raised to between about 300xc2x0 C. and about 450xc2x0 C. Subsequently an n-type film is grown on the substrate by directing an excimer pulsed laser beam onto a pressed powder pellet containing an n-type dopant element. Finally, a p-type ZnO film is grown on the n-type film by directing an excimer pulsed laser beam onto a pressed ZnO powder pellet containing a p-type dopant element to a p-type ZnO film having a net acceptor concentration of at least about 1018 acceptors/cm3.
The invention is further directed to a process for cleaning a substrate prior to growing a film on the substrate. A substrate is loaded into a chamber, the temperature is adjusted to between about 400xc2x0 C. and about 500xc2x0 C., and the chamber is filed with hydrogen to create a pressure between about 0.5 and about 3 Torr. The distance between a metal shutter in the chamber and the substrate is adjusted to between about 3 and about 6 centimeters and an excimer pulsed laser having an intensity between about 20 and about 70 mJ and a repetition of between about 10 to about 30 Hz is directed onto the shutter for a period of between about 5 and about 30 minutes to clean the substrate.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.